This invention relates to printing plates. More particularly, this invention relates to photopolymeric printing plates having a single photosensitive coating thereon composed of a photopolymerizable combination which includes a photopolymerizable pigment dispersion and a photosensitive lithographic diazonium resin and which is particularly well suited as a lithographic printing plate.
In lithographic printing plate technology, photosensitive compositions coated onto plates are needed which have desirable properties and which have substantially distinct physical characteristics before and after they are exposed to actinic radiation. The coated plate should be stable through time (i.e., not subject to changes in photospeed or ease of development) and should be non-tacky and mar resistant. The cured composition must be tough, resistant to solvents, and oleophilic. These are difficult properties to combine into one coating. This is especially so when the coating is applied to an electrochemically etched and anodized silicated aluminum substrate.
Patents describing lithographic printing plates utilizing a single, free radical photopolymerizable, organophilic layer which directly overlies the base sheet include, for example, U.S. Pat. Nos. 2,760,863; 3,468,311; 3,475,171; 3,512,971; 3,516,828; and 3,549,367. However, such known photopolymerizable layers characteristically provide poor adhesion (upon exposure and polymerization) to permanently hydrophilic, passivated substrates, such as silicated aluminum.
It will be appreciated that prior attempts have been made to obtain the above desirable properties by using multiple photosensitive coatings (generally two), on anodized aluminum substrates. For example, Larson U.S. Pat. No. 3,136,637 combines a first coating of diazo resin with a second overlayer of an organophilic resin. Thomas U.S. Pat. No. 3,808,004 combines a first coating of diazo resin with a second overlayer of a phenoxy cinnamate photopolymer. Bonham U.S. Pat. No. 3,905,815 comprises a base sheet bearing a thin layer of a water soluble, light sensitive diazo over which is provided a photopolymerizable layer.
Golda U.S. Pat. No. 4,104,072 produces a water developable plate by coating a first layer of a water soluble, light sensitive substance and a second overlayer of a water insoluble, ink receptive, photosensitive resin composition. Patrellis et al. U.S. Pat. No. 4,316,949 discloses a long running printing plate having a first layer of an oligomeric diazonium resin and a second overcoat comprising an oligomer, polymeric binder, solvent soluble photosensitive oligomeric diazonium resin, and a free-radical photoinitiator system. Asano U.S. Pat. No. 4,588,669 discloses a printing plate comprising a photosensitive diazo layer on the substrate and a second overlayer of a photoreactive polyvinyl acetal resin containing an aromatic azide group.
Commonly assigned U.S. Pat. No. 4,886,731 the disclosure of which is incorporated herein by reference describes a photopolymeric plate comprising three photosensitive layers where a first layer of a water-soluble, light sensitive lithographic diazonium is coated with two additional layers each containing a solvent soluble light sensitive diazonium and photopolymerizable components.
Although the use of multiple photosensitive layers provides means for achieving some of the properties in a desired lithographic plate made on a silicated substrate, several disadvantages still remain. These disadvantages include at least the following:
1. Multiple coatings require more sophisticated and complex manufacturing techniques.
2. Normally a water soluble diazo is coated first since the other overlayers are generally coated from a solvent. Water based diazo coatings (even when overcoated) tend to be less stable on silicated substrates then solvent soluble photosensitive coatings coated on the same silicated substrates.
3. Aqueous based developers are difficult to use on multiple coated plates unless the upper layer is permeated by the developer. However, if water tends to permeate the unexposed plate, then the first layer aqueous diazo coating is subject to degradation by atmospheric shelf life conditions. This again reduces the plate stability.
4. Very durable printing plates are difficult to prepare with a multiple coated silicated, grained and anodized plate, since adhesion between the several layers is crucial. It is preferable, especially with photopolymer type plates, to coat the photopolymer composition directly into the silicated, anodized grain on the aluminum.
5. The reproduction quality of plates produced with multiple photosensitive coatings can be poor unless the photospeed to each layer is properly matched. This can be difficult to achieve without compromising other plate qualities and difficult to maintain under normal manufacturing circumstances.